Referring to FIG. 1, a known example digital communication system 100 typically includes a base transceiver station (BTS) 102 that communicates with a communication network and one or more mobile units 104. The communication between the BTS 102 and the mobile unit 104 may be carried out using communication techniques such as code-division multiple access (CDMA), time-division multiple access (TDMA), and the like.
As will be readily appreciated by those having ordinary skill in the art, digital communication systems, such as that of FIG. 1, convert analog signals, such as voice signals, into digital audio signals, which are represented by bits. The digital audio signals are transmitted on a high frequency (HF) signal, e.g., 1800 megahertz (MHz), using digital modulation techniques such as phase shift keying (PSK), quadrature phase shift keying (QPSK), differential quadrature phase shift keying (DQPSK), and the like.
For example, with reference to FIG. 1, the BTS 102 may receive an analog voice signal from the network, wherein the voice signal is to be transmitted from the BTS 102 to the mobile unit 104. The BTS 102 samples the analog voice signal to create a corresponding digital audio signal and broadcasts the digital audio signal to the mobile unit 104 on an analog HF signal.
The mobile unit 104 typically includes a receive lineup that converts the received analog HF signal to one or more intermediate frequency (IF) signals, before converting the received signal to the digital domain (i.e., a digital signal) for processing. After the digital domain processing is complete, the digital audio signal is converted into an analog signal for presentation to a user via a speaker.
One known example receive lineup is a low-IF receive lineup, which uses an analog front end to convert an HF signal into a low-IF signal for further processing. It is a known problem that analog front ends output an undesired direct current (DC) component in addition to the received signal. The DC component may be dynamic and/or static in nature. Often times, the early stages of a receiver lineup remove the static DC component, leaving only the dynamic DC component. It is desirable to remove the DC component because the DC component reduces the dynamic range of analog-to-digital (A/D) converters, as well as degrades the performance of other components in the receive lineup. Additionally, the DC component appears as a noise contribution in detection phase of reception, thereby generating undesirable artifacts that make signal detection difficult. In particular, the DC component will appear as a tone after further mixing or down-conversion to zero-IF.
Conventional techniques for eliminating the DC component in low-IF receivers require the calculated estimation of the phase and amplitude of the tone generated by mixing the DC. This is a computationally intensive process. Other non-computationally intensive techniques pass received signals through sharp cutoff (i.e., multiple pole or high order) high-pass filters to filter out the DC component before subsequent down conversion from low-IF to zero-IF. However, due to their sharpness, such filters exhibit a non-linear phase response and large group delays at frequencies close to DC. In such systems, one approach to reducing group delay is to reduce the filter sharpness or order. However, less sharp or lower order filters attenuate useful signal energy (e.g., non DC components) and, therefore, adversely affect the signal-to-noise ratio (SNR) of the receive lineup.
Other existing solutions for addressing residual DC in a low-IF system perform DC correction on data prior to receiving an actual data burst. Such solutions are premised on the assumption that the DC signal does not change significantly during data burst reception. This is not always an accurate assumption because the DC signal may be dynamic and, therefore, based on a number of factors such as signal type, temperature, and mobile unit environment.